1. Field of the Invention
The present invention relates to an insulated wire and a coil using the same, more particularly, to an insulated wire and a coil using the same, to be used in motors and the like.
2. Description of the Related Art
An electrical equipment such as motor typically comprises a coil. A coil in motors is formed with using an insulated wire, and is formed by winding the insulated wire around a core of the motor, or joining the insulated wires together by welding or the like. The insulated wire comprises an insulative coating (insulating layer) on an outer periphery of a conductor. The insulating layer is formed by applying an insulative varnish containing a resin component dissolved in an organic solvent to the conductor, and baking the conductor with the insulative varnish.
Various characteristics such as mechanical characteristics and heat resistance have been required for the insulating layer of the insulated wire. As one of insulating layers satisfying the aforementioned characteristic requirements, an insulating layer using polyimide resin has been known. The polyimide resin is formed by imidization by heating polyamide acid (polyamic acid), which is synthesized from carboxylic anhydride and diamine. For example, JP-A 9-106712 discloses polyimide resin formed from polyamic acid, which is synthesized from, e.g., pyromellitic dianhydride (PMDA) as carboxylic anhydride and 4,4′-diaminodiphenyl ether (ODA) as diamine.
As well as the mechanical characteristics and heat resistance, a high partial discharge inception voltage (PDIV) is required for the insulating layer. The “partial discharge” is a phenomenon that the electric discharge occurs due to the electric charge concentrated at a small gap between adjacent insulated wires when voltage is applied to the conductor. The partial discharge inception voltage (Hereinafter also referred to as “PDIV”) means an applied voltage when the partial discharge starts to occur. The occurrence of the partial discharge does not cause the insulation breakdown immediately. The insulating layer is however eroded gradually by the partial discharge occurred therein, which eventually causes the insulation failure. In an insulating layer with a low partial discharge inception voltage (PDIV), the partial discharge is likely to occur at lower voltage, so that high PDIV is required in the insulating layer.